Do not validate xmlsec output for encrypt/decrypt, these operations generate no output.

Decrypting an encrypted response message with xmlsec generates no output and so this is treated as a failure.

e.g. this command produces no output even though it succeeds in decrypting the XML:
    /usr/bin/xmlsec1 --decrypt --privkey-pem ~/saml.key --id-attr:ID EncryptedKey --output /tmp/dec.xml saml-response-encrypted.xml

So we pass validate_output=False to the encrypt and decrypt operations.